It is in Chapter 2 that David describes to us the house in which he lives and the signs that you refer to. The signs are placed in the biggest room - the main living room of the family, and David comments on how it appears:

The nearest approach to decoration was a number of wooden panels with sayings, mostly from Repentances, artistically burnt into them. The one of the left of the fireplace read: ONLY THE IMAGE OF GOD IS MAN. The one on the right: KEEP PURE THE STOCK OF THE LORD. On the opposite wall two more said: BLESSED IS THE NORM, and IN PURITY OUR SALVATION. The largest was the one on the back wall, hung to face the door which led to the yard. It reminded everyone who came in: WATCH THOU FOR THE MUTANT!

I wouldn't necessary argue that these signs are examples of foreshadowing, as they don't really predict any particular events. What they do indicate is the kind of community that Waknuk is - one that always has its eyes open for deviants or "mutants" and is determined to purge them from the face of the earth. This, you could argue, foreshadows the zealous manner in which the community punish Sally and Katherine and then hunt Rosalind, David and Petra.